1. Field of the Invention
The present invention relates to a reflector which is placed on the back surface of a reflection-type display utilizing external light as a light source for lighting, such as a reflection-type liquid crystal display.
2. Related Art
Heretofore, there have been known reflection-type displays which have no exclusive light source but utilize external light as a light source, such as reflection-type liquid crystal displays. Unlike transmission-type displays, the reflection-type displays utilize light on the observation side, which is allowed to reflect on a reflector placed on the back surface of their display elements. Since the reflection-type displays effectively utilize external light, they are advantageous in that no electric power is required for a light source and that images displayed can be brightly observed when external light is bright.
However, a conventional reflector which is placed on the back surface of a display element such as a liquid crystal display element when a reflection-type display is fabricated has not necessarily been satisfactory in properties. FIG. 6 shows one example of the reflection-type liquid crystal display. This display is composed of a liquid crystal display element 20, a surfacing material 30 having antiglaring properties or antireflection effect, which is provided on the observation-side surface of the liquid crystal display element 20 so as to prevent this surface from glaring, and a matted reflector 40 having a matted plane reflecting surface with fine roughness, provided on the back surface of the liquid crystal display element 20. As the matted reflector 40 is used a reflector in which a layer of a metal such as aluminum is provided, by means of vacuum deposition or the like, as a light-reflecting layer on a matted polyethylene terephthalate film whose surface itself is matted, or on a polyethylene terephthalate film having thereon a matted layer formed by applying a coating liquid containing fine particles.
However, as can also be noticed from FIG. 6, although incident light reflected on the reflector serves as display light (reflected light) useful for displaying images, a part of the incident light reflects on the surface of the surfacing material or of the liquid crystal display element, and becomes useless surface-reflected light. Moreover, the reflecting surface of the reflector and that of the surfacing material are parallel with each other, so that the angle .alpha. of reflection of the display light is equal to the angle .beta. of reflection of the surface-reflected light. The display light and the surface-reflected light thus become parallel with each other. It is noted that the angle of reflection herein does not mean an angle of reflection formed by light with a surface on which the light actually reflects, but means an apparent angle of reflection formed by light with the image-displaying surface of a display. Therefore, images displayed on the image-displaying surface can be most brightly seen when observed from the direction in which display light travels. However, the surface-reflected light also travels in this direction, so that the external light source is seen on the image-displaying surface due to the surface-reflected light. Therefore, the image-displaying surface is most glaring when observed from this direction. Thus, the direction from which images can be observed most brightly is the direction from which the images cannot be observed without difficulty due to the surface-reflected light with highest intensity.
The above problem can be solved when the angle of reflection of display light and that of reflection of surface-reflected light are made different from each other. This can be attained, for example, by a reflection-type display shown in FIG. 7. The reflection-type display shown in this figure is the same as the reflection-type display shown in FIG. 6 except that the matted reflector 40 is replaced with a serrated reflector 50. Namely, this reflection-type display is composed of a liquid crystal display element 20, a surfacing material 30 having antiglaring properties or antireflection effect, which is provided on the observation-side surface of the liquid crystal display element 20 so as to prevent this surface from glaring, and a reflector 50 having a reflecting surface whose cross section is serrated, the shape of the serration being a scalene triangle, provided on the back surface of the liquid crystal display element 20. On the reflecting surface of the reflector 50, a light-reflecting layer is formed by, for example, vacuum-depositing a metal such as aluminum.
As can also be understood from FIG. 7, when the serrated reflector 50 is used, the reflecting surface of the reflector is not parallel with the image-displaying surface of the display. As a result, the angle .alpha.1 of reflection of display light and the angle .beta.1 of reflection of surface-reflected light become different from each other, and the display light and the surface-reflected light travel in different directions. However, the reflecting surface of the serrated reflector 50 is a mirror surface, so that light does not diffuse on the surface of the reflector at all. Therefore, such a problem is brought about that the angle of directions in which images displayed can be brightly observed is limited to an extremely narrow range depending upon the direction of external light.
FIG. 8 is a characteristic diagram of luminance distribution by angle (the direction in which reflected light mainly travels is taken as angle 0*), conceptually showing the luminance distribution of reflected light in reflection-type display shown in FIG. 6 and that of reflected light in the reflection-type display shown in FIG. 7. The luminance distribution 64 obtained when the matted reflector 40 is used is broad, and this shows that acceptable brightness can be obtained in a wide angle. On the other hand, with respect to the luminance distribution 65 obtained when the serrated reflector is used, the peak luminance is greater but the luminance distribution is narrower than those obtained when the matted reflector 40 is used. This means that the luminance lowers drastically with a slight change in the direction of observation and that the optimum visual angle is narrow.